An Instrument Current Transformer is a transformer designed to provide a current in its secondary coil proportional to the current flowing in its primary coil. Instrument transformers are used for measuring voltage and current in electrical power systems, and for power system protection and control. Where a voltage or current is too large to be conveniently used by an instrument, it can be scaled down to a standardized, low value. Instrument transformers isolate measurement, protection and control circuitry from the high currents or voltages present on the circuits being measured or controlled.
In the case of the electric utility and the electric utility industry, the meter is the cash register for all revenue. For the past 110 years meter manufacturers, utilities and their customers have continued to standardize the art of revenue metering. The Electric Utilities Industry has continued to update both their metering practices and metering standards, because of the innovation of new products and meters. The industry's standards and practices go back 100 years; the primary change in metering is the functionality and accuracy of the meter as well as the ability to meter different forms. The meter's physical size and shape has not really changed in 100 years because the infrastructure (meter socket) has not changed. The transformer is still an integral part of a transformer rated metering system, and there are no plans to replace the transformer and build a meter that can withstand high current and voltage conditions
The basic electromechanical design of the transformer (core, primary and secondary winding) has not changed over time. The physical structure of the transformer has been standardized for mounting purposes and the ability to endure extreme conditions that could cause internal damage have slightly improved over the years based on new insulating material. Today only the second generation of instrument transformers are available and there haven't been new developments in this area for 25 years.
Instrument Transformers are constructed to comply with the following standards:
Canadian Standards Association (CSA)
American National Standards Institute (ANSI)
The Electrical and Electronic Manufacturers Association of Canada (EEMAC)
AEIC—EEI—NEMA Standards for Instrument Transformers
UL and Measurement Canada
Instrument Transformers are constructed to comply to the following tests:
Continuous Thermal Rating: 200% of rated current
One second thermal rating: 50 times normal current
Impulse Test: 10 kv full wave
60 Hz insulation test: Primary-4000V for one minutes
The test switch is utilized in series between the transformers and the meter. The test switch is comprised of current shorting switches and voltage switches. The primary function of the test switch is to short the secondary winding of the current transformer. Often a meter is changed, and when we remove the meter (without using the current shorting switch), we open the secondary circuit. When the secondary circuit is left open, voltage builds up in the current transformer. The high voltage build up can cause damage to the current transformer and could effect accuracy or ultimately catch fire. The secondary function of the test switch is to provide a means to test the meter and or system A socket meter plugs into a socket, an adapter or a microswitch. These North American standard devices are designed to accept a socket meter. Small copper lugs extrude out from the bottom of the base of the meter, and these lugs penetrate the jaws of the socket. The variation of the number of lugs and their location on the base of the meter reflects the type of service being metered. Once the socket meter is installed and the lugs have penetrated the jaws of the socket, the outer edge flange of the meter is now aligned with the collar of the meter cabinet. The collar is on the outside face of a meter cabinet that contains a socket adapter for the meter to plug into. When the meter is installed the cabinet collar abuts against the meter flange. A meter ring is lowered over the collar and the flange, grasping both the collar and the flange and preventing them from separating. The meter can now be protected and secured to the cabinet by the use of a meter ring. The socket meter is the preferred meter of choice for most installations because the infrastructure is already present and the ease of removal. There are however different meter connection scenarios that include bottom connect meters, switchboard meters and European design meters etc. The design of the TIMS will accommodate most meter installation connections.
To appropriately meter, protect and secure a polyphase meter installation whose load is greater than 200 amps, electric utilities require (3) separate current transformers (one for each phase), a test switch and a meter socket arrangement for socket meters. These devices are installed by utility personnel in several metal cabinets at the site of the electric load. The transformers are located in a transformer cabinet complete with mounting bracket (including Cooper CT Mounting bracket). The test switch and meter mounting and attachment devices are located in a metering cabinet. Today these devices are separate items that are required to be wired and interconnected to each other according to metering standards and utility practices and meter type.
The cost of ownership for a transformer rated metering system is relatively high in comparison to a self contained system. Before a trained utility metering personnel enters a transformer rated metering site he must be trained and educated on a regular basis. Inventory of over 30 items associated with a transformer rated metering system used in conjunction with installation must be stocked and stored. Therefore the financial costs to install, secure, and maintain an accurate metering system is high. The cost of metering errors and the cost of stolen power must also be considered.
Accuracy of the system is critical for appropriate billing for both the customer and the utility. Testing of the accuracy of the transformer is performed during manufacturing and testing of the meter is periodically done, but the transformer metering system is rarely tested as a system. There are a number of factors that could influence the accuracy of the system; one factor that must be taken into consideration is the burden of the system (load; distance between transformers and the meter and the meter itself have a burden). The design of the core is singularly responsible for accuracy and burden characteristics. Commissioning, performed by the testing of the primary and comparing results with both the secondary and meter is rarely done. The best way to test the accuracy is to test the system as a singular metering entity.
Management and security of the system is maintained by providing a means of detecting any intrusion or disruption of the system. Utilities use a number of seals for each transformer, test switch, cabinet, and meter for security purposes. The intention is not to prevent intrusion, but detect infringement.
Presently Electric utilities use both bar-type and window type instrument transformers for low voltage applications. Each transformer must not only comply with Standards and Tests but must also meet accuracy characteristics for revenue purposes. Each manufacturer's transformer must meet certain physical characteristics for mounting, markings and identification. Each transformer must endure testing without physical or accuracy deterioration.
Presently transformers are installed in a cabinet on a mounting bracket. At times utilities specify other requirements such as the Cooper CT Mounting bracket, yet still installed in a cabinet. Typically utilities do not purchase cabinets, only the metering equipment. Other installation scenarios would include a Bus-bar arrangement. The cabinets and power distribution are typically located in a electric room.